1. Field of the Invention
The present invention relates to a process and an apparatus for manufacturing a pleated filter insert, which is made of thermoplastic material. The filter material is to be placed between the inflow and outflow sides of a filter, and includes a plurality of elongated bulges or ribs in at least the filter pleats which are open in the direction of the outflow side of the filter. The ribs are shaped from the filter material itself and maintain the filter walls at a distance from each other.
2. Description of Background and Relevant Information
A process and apparatus of the type described above is disclosed in U.S. Pat. No. 3,531,920, wherein the material, which may be a thermoplastic material, is fed through a roller to a press. The press includes two heated cylinders, which are rotatable in opposite directions, and are provided with meshing raised portions and recesses intended for the shaping of ribs and recesses in the filter material. Between these two heated cylinders, the filter material is provided with the elongated ribs and recesses, which space the filter pleat walls apart, as well as with crosswise grooves which facilitate the shaping of pleat edges. In this process, the filter material is permanently shaped by pressing between the two heated cylinders. However, the structure of the filter material in the pressed area is also changed by the heated pressing. For this reason, the filter material which is permanently shaped by this process cannot retain its original filtering properties in areas which are important for filtering. Therefore, it is necessary to use a larger filter insert, which entails higher costs than would be required for a filter insert with totally usable filter walls, to obtain the same filter effect.
Another apparatus for manufacturing a pleated filter insert is described in French Patent No. 2,273,657. In this apparatus, a stretchable filter material is first provided with alternate convex and concave longitudinal grooves by changing the structure of the material through stretching of the filter material between two rollers. In a second step, the filter material is folded transverse to the longitudinal grooves in order to obtain pleated filter inserts. In order to be able to fold the filter material transverse to the longitudinal grooves, it is necessary to use a stretchable filter material. However, most filter materials used are not stretchable, or at least change the filtering properties of the filter material when stretched. For this reason, this apparatus is only usable in connection with particular stretchable filter materials. Furthermore, the pleat edges, which occur when pleating is transverse to the longitudinal grooves, have unwanted irregularities which, at a minimum, make cleaning of the filter insert harder.